The present invention relates generally to electrochemical devices and relates more particularly to electrochemical devices of the type comprising a solid proton exchange membrane.
Electrochemical devices of the type comprising a solid polymer electrolyte membrane (PEM) are well-known, such electrochemical devices finding applications as, for example, fuel cells, electrolyzers, sensors, gas concentrators, gas compressors, supercapacitors, ultracapacitors and industrial electrochemical process units. A common type of solid polymer electrolyte membrane that is used in electrochemical devices consists of a homogeneous perfluorosulfonic acid (PFSA) polymer, said PFSA polymer being formed by the copolymerization of tetrafluoroethylene and perfluorovinylether sulfonic acid. Such polymers are good conductors of ions but poor conductors of electrons. See e.g., U.S. Pat. No. 3,282,875, inventors Connolly et al., issued Nov. 1, 1966; U.S. Pat. No. 4,470,889, inventors Ezzell et al., issued Sep. 11, 1984; U.S. Pat. No. 4,478,695, inventors Ezzell et al., issued Oct. 23, 1984; U.S. Pat. No. 6,492,431, inventor Cisar, issued Dec. 10, 2002, all of which are incorporated herein by reference. A commercial embodiment of a perfluorosulfonic acid polymer PEM is available from DuPont (Wilmington, Del.) as NAFION® PFSA polymer.
Typically, the solid polymer electrolyte membrane is sandwiched between a pair of electrodes at the membrane interfaces on which desired electrochemical reactions take place, one of the electrodes functioning as an anode and the other of the electrodes functioning as a cathode. A first catalyst layer is typically positioned between the anode and the membrane, and a second catalyst layer is typically positioned between the cathode and the membrane, the catalyst layers either being formed as part of the electrodes or being applied to the solid polymer electrolyte membrane. The combination of the membrane, the catalysts and the electrodes is commonly referred to in the art as a membrane electrode assembly (MEA).
Where the electrochemical cell is used as a fuel cell to generate electricity, a fuel is supplied to the anode, and an oxidizing agent is supplied to the cathode. The electrodes are connected electrically to a load, such as an electronic circuit, by an external circuit conductor. Oxidation of the fuel at the anode produces electrons that flow through the external circuit to the cathode producing an electric current. The electrons react with an oxidant at the cathode. In theory, any substance capable of chemical oxidation that can be supplied continuously to the anode can serve as the fuel for the fuel cell, and any material that can be reduced at a sufficient rate at the cathode can serve as the oxidant for the fuel cell.
In one well-known type of fuel cell, sometimes referred to as a hydrogen fuel cell, gaseous hydrogen serves as the fuel, and gaseous oxygen serves as the oxidant. (In another well-known type of fuel cell, sometimes referred to as a direct methanol fuel cell, liquid methanol or an aqueous solution of methanol is used instead of hydrogen as the fuel.) The electrodes in a hydrogen fuel cell are typically porous to permit the gas-electrolyte junction to be as great as possible. At the anode, incoming hydrogen gas ionizes to produce hydrogen ions and electrons. Since the electrolyte is a non-electronic conductor, the electrons flow away from the anode via the external circuit, producing an electric current. At the cathode, oxygen gas, either from a pure supply or from air, reacts with hydrogen ions migrating through the electrolyte and the incoming electrons from the external circuit to produce water as a byproduct. The overall reaction that takes place in the fuel cell is the sum of the anode and cathode reactions, with part of the free energy of reaction being released directly as electrical energy and with another part of the free energy being released as heat at the fuel cell. Often, a number of fuel cells are assembled together in order to meet desired voltage and current requirements. One common type of assembly, often referred to as a bipolar stack, comprises a plurality of stacked fuel cells that are electrically connected in series in a bipolar configuration.
An electrolyzer is similar in structure to a fuel cell but functions essentially in reverse to a fuel cell. Consequently, in the case of a water electrolyzer, water and electricity are provided, and molecular hydrogen and molecular oxygen are produced. In another common type of electrolyzer, water and sulfur dioxide are provided, and sulfuric acid and hydrogen gas are produced.
Most fuel cells are run using a finite quantity of fuel, the fuel typically being withdrawn from a storage vessel as needed. For example, in the case of a hydrogen fuel cell, hydrogen gas is typically stored in and withdrawn from a hydrogen storage tank. As can be appreciated, if fuel is withdrawn from a storage vessel, and the fuel is not replenished thereafter in some manner, then eventually there will be no fuel left for the fuel cell to operate. A regenerative fuel cell system addresses this problem by including equipment that may be used to regenerate fuel for the fuel cell. For example, in the case of a hydrogen fuel cell system, the equipment for regenerating fuel may include an electrolyzer that is run to convert water into oxygen gas and hydrogen gas. The electrolyzer may be operated using solar, wind or geothermal energy so as not to deplete the electrical energy produced by operation of the fuel cell. In this manner, a regenerative fuel cell system may be used in a fashion similar to a rechargeable battery, with the electrolyzer being run to store energy and with the fuel cell being run to generate electrical current. A regenerative fuel cell system may include separate electrolyzer and fuel cell units or may include a bifunctional unit that may be alternately operated either as an electrolyzer or as a fuel cell. In those instances in which a bifunctional unit is used, the system is typically referred to as a unitized regenerative fuel cell system. Regenerative fuel cell systems may be either closed-loop, in which case the quantities of fuel, oxidant and products are limited, or open-loop, in which case the quantities are unlimited.
Additional background information relating to regenerative fuel cell systems may be found, for example, in the following patents and publications, all of which are incorporated herein by reference: U.S. Pat. No. 6,887,601 B2, inventors Moulthrop, Jr. et al., issued May 3, 2005; U.S. Pat. No. 6,838,205 B2, inventors Cisar et al., issued Jan. 4, 2005; U.S. Pat. No. 6,833,207 B2, inventors Joos et al., issued Dec. 21, 2004; U.S. Pat. No. 3,981,745, inventor Stedman, issued Sep. 21, 1976; Giner et al., “Fuel Cells As Rechargeable Batteries,” Proceedings NATO-ARW, Kiev 5/95 (Kluwer, Dordrecht, 1/96) pp. 215-232; Burke, “High Energy Density Regenerative Fuel Cell Systems for Terrestrial Applications,” IEEE AES Systems Magazine, 23-34 (1999); and Ioroi et al., “Thin film electrocatalyst layer for unitized regenerative polymer electrolyte fuel cells,” Journal of Power Sources, 112:583-7 (2002).
Problems that are commonly encountered in electrochemical cells of the type comprising solid polymer electrolyte membranes include the removal of products from the membrane electrode assembly or the continued supply of reactants to the membrane electrode assembly. For example, in the case of a hydrogen fuel cell, water tends to accumulate on the cathodic catalyst, where water is produced. This is problematic because the accumulated water often impedes the delivery of additional reactant gases to the catalyst. This is generally addressed by operating one or both of the feed gases at high excess stoichiometries and separating the product water. However, this approach is not always feasible, such as when the quantities of gases are limited or when the fuel cell is a dead-end fuel cell (i.e., a fuel cell having a gas inlet but no gas outlet). Also, in the case of a water electrolyzer, water is typically fed to the electrolyzer at either the oxygen or hydrogen electrode. This is typically done at a high stoichiometric ratio to cool the stack and to ensure the utilization of the entire surface area. The evolved product gas and excess water then need to be separated, often under high pressure, with recovery of the water.
In U.S. Pat. No. 4,729,932, inventor McElroy, which issued Mar. 8, 1988, and which is incorporated herein by reference, there is described a fuel cell comprised of individual fuel cells each having a barrier plate bounding the individual fuel cell, an anode chamber, a cathode chamber, a solid polymer electrolyte membrane having the cathode and anode intimately contacting opposite surfaces of the membrane and separating the anode and cathode chambers, and a gas/water separator which includes a porous hydrophilic structure associated with the cathode chamber and bounding the other side of the individual fuel cell. The porous hydrophilic structure is permeable to liquid but impermeable to gas flow at gas pressures below a “bubble pressure P” which is related to the pore size of the structure by the relationship P=(2×(surface tension))/(pore radius for individual circular pores).
In U.S. Pat. No. 3,418,168, inventor Wentworth, which issued Dec. 24, 1968, and which is incorporated herein by reference, there is disclosed a microporous sheet for use as a wick in fuel cells, the microporous sheet being composed of a hydrophilic filler and a vinyl chloride/acrylonitrile copolymer.
In U.S. Pat. No. 6,171,720 B1, inventors Besmann et al., which issued Jan. 9, 2001, and which is incorporated herein by reference, there is disclosed a combination bipolar plate/diffuser fuel cell component. The component includes an electrically conducting solid material having a porous region having a porous surface; and a hermetic region, the hermetic region defining at least a portion of at least one coolant channel, the porous region defining at least a portion of at least one reactant channel, the porous region defining a flow field medium for diffusing the reactant to the porous surface.
Other patents of interest include the following, all of which are incorporated herein by reference: U.S. Pat. No. 6,811,905 B1, inventors Cropley et al., which issued Nov. 2, 2004; U.S. Pat. No. 6,808,838 B1, inventor Wilson, which issued Oct. 26, 2004; U.S. Pat. No. 4,678,724, inventor McElroy, which issued Jul. 7, 1987; and U.S. Pat. No. 4,543,303, inventors Dantowitz et al., which issued Sep. 24, 1985.